Tetrachloropyridine is an important commercial product useful in preparation of insecticide formulations. It has been prepared commercially by heating pentachloropyridine in an alchohol medium (e.g., isobutanol and water azeotropic mixture) in the presence of zinc and HCl. The commercial preparation in alcohol normally results in maximum yields and conversions of about 78 and 80 percent, respectively, and side-products tri- and di-chloropyridine are wasted.
The prior process which employs alcohol as a reaction medium has several deficiencies which are best avoided, if possible. One deficiency is that alcohol is flammable and toxic and presents hazards to persons working on or near the process. Another deficiency is that it is virtually impossible in large commercial operations to prevent spilled or escaped alcohol from polluting the air or nearby drainage conduits. A further deficiency is that alcohol is miscible, to a significant extent, with the starting material (pentachloropyridine or PCP), with the side-products, and with the product of the reaction (sym-tetrachloropyridine or s-TCP). The metal salt, e.g., ZnCl.sub.2, which is produced as a by-product is more easily put to good use if it is not contaminated with alcohol. The side-products which are made in the process (such as, di- and tri-chloropyridine) and residual unreacted PCP are difficult to completely extract from alcohol.
The prior process referred to above can be graphically represented as follows: ##SPC1##
The embodiment illustrated here shows that pentachloropyridine (m.p. 124.degree. C) is converted to symmetrical-tetrachloropyridine (s-TCP) by reacting with Zn and HCl in iso-butanol carrier at about atmospheric pressure and at reflux temperature. ZnCl.sub.2 is formed and other by-products (not shown) which are formed are small amounts of unsymmetrical-tetrachloropyridine, trichloropyridine, and dichloropyridine. Pentachloropyridine is partly soluble in iso-butanol (about 31.4 gms./100 gms. of i-BuOH). Since the starting material (PCP) and the reaction products are somewhat soluble in the alcohol, complete extraction and separation of these materials are difficult and inefficient.
It can be readily understood that, aside from the need to obtain better-yields of sym-tetrachloropyridine and conversions in a commercial process, it is highly desirable and beneficial to provide a safer, more economical process, reduce the contaminants of the product and by-product, avoid environmental pollution, and to conserve hydrocarbons.
It is an object of the present invention to obtain better yields and conversions than are obtained by the prior method of using an alcohol reaction medium.
It is another object to provide a safer process than when alcohol is employed.
Another object is to provide a process which does not have the pollution problems associated with the use of alcohol and to obtain products and side-products which are not contaminated with alcohol.
Yet another object is to provide a process which conserves hydrocarbon values by avoiding the use of alcohol as a reaction medium.
These and other objects are obtained by the present invention as described hereinafter.
It has now been found that by employing an aqueous reaction medium, rather than an alcohol reaction medium, that a safer, ecologically beneficial process is obtained which, surprisingly and unexpectedly gives better conversions and better yields to the desired TCP and less side products, such as di- or trichloropyridine. The metal salt (e.g., ZnCl.sub.2) which is formed is more easily purified than when contaminated with alcohol, thus is more easily and economically put to further use, such as in preparing particulate metal for recycling in the process.